An Interconnect cable is the patch cable that goes between audio separates. For home use they typcially use "RCA" style connectors. The following post is aimed at those types of cables and not XLR style interconnects which can be found on some home audio components.
Go onto Craigslist, Nextdoor, Ebay, or Facebook Marketplace and look for RG6 75ohm coaxial cable. It's the cable that is run in most houses for cable TV. You might even have this stuff laying around your house in a box somewhere. Either way you can get it for pennies per foot if you look hard enough. There are versions with a solid copper core , or a copper clad steel core. For audio use, either is just fine, but as a personal preference I like to use solid copper core as it's better quality cable. However, it does cost just a bit more, but still cheaper than buying cables in a box store.
Next you'll need RCA compression fittings for RG6: Amazon Link
Then purchase a compression tool and coaxial cutter: Amazon Link
Follow the instructions that come with the tools and enjoy cables that outperform even those that are made of unicorn farts and fairy dust that people with too much money but little sense like to tout. These cables will also last forever as RG6 cable is very tough, especially the solid copper core version. I have cables that are north of 30 years old still in daily use.
The only issue with RG6, it is a stiff cable. So making short runs bend at sharp angles is not an easy thing to do. You will need to come up with a method to keep stereo pairs together, or just run them separately. Plus mark the cables as Left or Right channels. I use either colored heat shrink tubing, or electrical tape for this purpose.
